About one million years ago,the Ice Age began.The Ice Age was a long period of time in which four great glaciers(冰川) pushed southward to cover almost all the upper half of North America,and then melted away.Each glacier was a thick sheet of ice and snow that spread out from a center near what is now Hudson Bay in Canada.The winters were long,and the cool summers were too short to melt much of the ice and snow.The evergrowing sheet built up to a thickness of two miles at its center. As all glaciers do,these great glaciers slid(滑动). They pushed down giant trees in their paths and scd(刮削)the earth bare (光秃秃)of soil.Many animals moved farther south to escape.Others stayed and were destroyed. When winters of little snow came,the summer suns into the edges to the ice sheets.As the glaciers melted,rocks,soil and other things that had mixed with the ice and snow were left.New hills,lakes and rivers were formed. The last of the great glaciers began its melting about 11 000 years ago.Itsmelting formed the Great Lakes.These lakes are today little changed from their early sizes the glaciers.This is the Mississippi Missouri Ohio system.These rivers were miles wide at first.Through the years they settle into their present channels. The main idea of this passage is  ________ . A.the Ice Age was a long period of time B.great glaciers covered North America many years ago C.changes in climate helped to melt the glaciers D.how glaciers changed North America 2.The author states that all glaciers ________. A.are two miles thick           B.form frozen lakes C.are a million years old      D.move and slide 3.From the information in this passage we know that  ________. A.glaciers are destructive B.all glaciers in the world move southward C.the Mississippi Missouri Ohio systems is larger than it was before the Ice Age D.the Great Lakes are now smaller than they were before the Ice Age 4.The Ice Age lasted almost ____. A.1 000 years      B.100 years      C.1 000 000 years      D.11 000 years 5.In the last sentence,the word “their” refers to  ________ . A.lakes            B.rivers         C.glaciers             D.systems